Configuring HSRP (Hot Standby Routing Protocol)

Problem

Normally your client machines have one route off the network, (their default gateway). But what if that goes down? HSRP aims to solve this problem by assigning a ‘Virtual IP address’ to your default gateway (or default route). So that IP can be shared amongst two or more possible devices (routers, or layer 3 switches).

Above, we have a client 192.168.1.10 that has two possible routes off the network, (.254 and .253). We will setup a virtual IP of .250 and both routers can use that IP, (if they are the active gateway). Below is a brief overview of how to set it up.

Solution

Setting up HSRP

1. On the first router (Router0), add the standby IP address (192.168.1.250) the ‘1’ denotes the standby group (a number from 0 to 4096). It comes up as standby, then after it has checked (via multicast address 224.0.0.2 on UDP port 1985). It finds no other live HSRP devices using that IP address so it becomes ‘Active’.

4. That is HSRP configured! However there are a few changes you might want to make, for example, what if one router had a 100MB leased line, and the other was a 2MBADSL line, you would want the fastest one to be in use, (as long as it was up). To achieve that, give the router with the fastest connection a higher priority (you may notice above, that by default the priority is 100). Be aware, even if a router has the highest priority, it wont ‘seize’ the virtual IP, it just sits and waits until it’s available. For our 100MB and 2MB example that’s not good. We would want Router1 to seize the virtual IP as soon as it can. To do that we need to set it to preempt. (Note: This process is called ‘launching a coup’).

5. At this point it’s important to say, that in our scenario we would also need to setup a virtual IP for the ‘other side’ of the routers (i.e their GigabitEthernet 0/1 interfaces), or the remote client (172.16.1.10) would not be able to return our ‘pings’ or get any traffic back to us. So lets setup a virtual HSRP address on that side as well. Notice I just use another standby group number.

Note: To work the remote host 172.16.1.10 will need its default gateway changing to the HSRP Virtual IP of 172.16.1.250.

6. Finally we have set Router0 with the highest priority and set it to seize the virtual IP as soon as it can. But what if another interface on Router1 goes down? e.g. If the GigabitEthernet 0/1 interface were to go down, HSRP would not do anything because it’s tracking both the GigabitEthernet 0/0 interfaces, so communications would fail.

To solve the problem we need to tell it which interfaces to ‘Track’. In our example we need to track GigabitEthernet 0/1, if that goes down we need to give the virtual IP address to the standby ‘router’. This works because once we tell it to ‘track’ the GigabitEthernet 0/1 interface, if that were to fail it will DECREMEMT the routers priority by 10. So for Router0 its priority would drop to 95, this is five less than the default value of 100 (on Router1). But Remember, at the moment that fail-over would still fail, unless you allow Router1 to preempt and launch a coup.